Topic 5 Practice Paper: Chemical energetics — question paper

Section A: Multiple-choice questions[10 marks]

Choose one answer for each question. Each question is worth 1 mark.

1. 1

   Which row correctly describes an exothermic reaction?

   1. AIt takes in energy; the surroundings cool down
   2. BIt gives out energy; the surroundings warm up
   3. CIt gives out energy; the surroundings cool down
   4. DIt takes in energy; the surroundings warm up
   ( )
2. 2

   In an endothermic reaction, thermal energy is

   1. Areleased to the surroundings, which get hotter
   2. Bnot transferred at all
   3. Creleased to the reactants
   4. Dtaken in from the surroundings
   ( )
3. 3

   Which statement about bond breaking and bond forming is correct?

   1. ABreaking bonds takes in energy; forming bonds releases energy
   2. BBreaking bonds releases energy; forming bonds takes in energy
   3. CBoth breaking and forming bonds release energy
   4. DBoth breaking and forming bonds take in energy
   ( )
4. 4

   What is meant by the activation energy of a reaction?

   1. AThe energy given out when the products form
   2. BThe energy stored inside the reactant particles
   3. CThe least energy colliding particles need in order to react
   4. DThe energy difference between the reactants and the products
   ( )
5. 5

   On a reaction pathway diagram, which feature shows that a reaction is exothermic?

   1. AThe peak is high above the reactants
   2. BThe products are lower in energy than the reactants
   3. CThe products are higher in energy than the reactants
   4. DThe curve is drawn as a straight line
   ( )
6. 6

   Bond energies in kJ per mol: H–H 436, O=O 498, O–H 464. For 2H₂ + O₂ → 2H₂O, what is the energy taken in to break all the bonds in the reactants?

   1. A872 kJ
   2. B934 kJ
   3. C1366 kJ
   4. D1370 kJ
   ( )
7. 7

   Why does raising the temperature usually speed up a reaction?

   1. AA greater fraction of collisions has enough energy to react
   2. BThe particles become smaller and fit together more easily
   3. CThe activation energy of the reaction is lowered
   4. DThe products are removed more quickly
   ( )
8. 8

   A temperature rise is recorded when two solutions are mixed in a polystyrene cup. Which conclusion is correct?

   1. AThe reaction is endothermic and the mixture lost energy
   2. BNo reaction has taken place
   3. CThe reaction is exothermic and transferred energy to the mixture
   4. DThe mixture absorbed energy from the air
   ( )
9. 9

   A reaction has an overall energy change of −250 kJ per mol. What does this tell you?

   1. AThe reaction is endothermic and takes in 250 kJ per mol
   2. BThe reaction is exothermic and gives out 250 kJ per mol
   3. CThe reaction needs 250 kJ per mol to start
   4. DThe reaction releases 250 kJ per mol only when heated
   ( )
10. 10

    The energy released when new bonds form is greater than the energy taken in to break the old bonds. What is the result?

    1. AThe reaction is endothermic
    2. BThe temperature of the surroundings falls
    3. CThe activation energy is zero
    4. DThe reaction is exothermic
    ( )

Section B: Structured questions[20 marks]

Answer all questions in the spaces provided.

1. 11

   A student adds a spatula of magnesium powder to dilute hydrochloric acid in a beaker and stirs the mixture. The thermometer reading rises from 21 °C to 34 °C.

   1. (a)

      Name the type of reaction taking place and explain your answer using the thermometer readings.

      [2]
   2. (b)

      State one practical change the student could make to the apparatus to reduce heat loss to the room.

      [1]
   3. (c)

      The experiment is repeated using half the mass of magnesium but the same volume of the same acid. Explain the effect this has on the temperature rise.

      [1]
2. 12

   The equation for the reaction of methane with oxygen is CH₄ + 2O₂ → CO₂ + 2H₂O. A reaction pathway diagram for this reaction is drawn with the products lying below the reactants.

   1. (a)

      State what the diagram shows about the total energy of the products compared with the reactants.

      [1]
   2. (b)

      Explain, in terms of bonds, why the overall energy change of this reaction is negative.

      [3]
   3. (c)

      A flame is needed to start the reaction even though it is exothermic. Use the idea of activation energy to explain why.

      [1]
3. 13

   Bond energies in kJ per mol: N≡N 945, H–H 436, N–H 391. These are used to work out the energy change for the reaction N₂ + 3H₂ → 2NH₃.

   1. (a)

      Work out the total amount of energy that has to be supplied to separate the atoms in the reactant molecules.

      [2]
   2. (b)

      Work out the total amount of energy released when the bonds in the product molecules form.

      [1]
   3. (c)

      Calculate the overall energy change and state whether the reaction is exothermic or endothermic.

      [2]
4. 14

   A student investigates three reactions and records the temperature of each mixture at the start and at the end.

   Temperature records for three reactions

   Reaction	Starting temperature / °C	Final temperature / °C
   P: burning ethanol	20	31
   Q: dissolving ammonium nitrate in water	20	14
   R: adding zinc to copper sulfate solution	20	27

   1. (a)

      State which of reactions P, Q and R are exothermic and give a reason for your answer.

      [2]
   2. (b)

      Explain, in terms of the surroundings, why the temperature falls during reaction Q.

      [2]
   3. (c)

      For reaction R, calculate the temperature change and state whether the products are at a higher or a lower energy than the reactants.

      [2]